The instant invention relates to control circuits for vehicle passenger restraint systems, such as air bags, comprising a plurality of acceleration-responsive sensors.
Known air bag passenger restraint systems employ a control circuit wherein a power supply applies a voltage across a firing path which includes in series an explosive squib and a plurality of normally-open acceleration sensors, each of the sensors being shunted by a resistor having a nominal resistance substantially greater than the internal resistance of the squib. In normal operation, a small current flows through the circuit while the sensors remain in the normally-open position. The closure of the sensors upon collision or marked deceleration of the vehicle generates a significant rise in the current flowing through the squib, thereby "firing" the squib and deploying the air bag. See, e.g., U.S. Pat. No. 4,695,075, issued Sept. 22, 1987 to Kamiji et al.
Significantly, if any of the sensors fails to close in response to the collision or marked vehicle deceleration, e.g., the sensor becomes stuck in the "open" position, the current flowing through the squib will not rise upon such collision or marked vehicle deceleration and, thus, the air bag will not deploy, thereby placing the vehicle passengers at significant risk. Alternatively, in the event that any of the sensors fails with a propensity to close, e.g., becomes stuck in the closed position, the prior art teaches the disabling of the entire control circuit to prevent the unintentional or premature triggering of the passenger restraint, once again placing the passengers at risk. See, e.g., U.S. Pat. No. 3,889,232, issued June 10, 1975 to Bell, wherein the control circuit shuts down when one sensor closes without the corresponding closing of the other sensor.
Frequently, such known firing circuits comprise two acceleration sensors, one of which closes in response to an acceleration input which is lower than the acceleration input required to close the other sensor, whereby one sensor is essentially "armed" by the other sensor. In a variation on this theme, U.S. Pat. No. 3,780,314, issued Dec. 18, 1973 to Inose et al. teaches a control circuit wherein a low acceleration threshold "arming" sensor closes to energize a coil integral with a normally-open, high-threshold "crash" sensor. The energized coil generates a magnetic field which acts to reduce the magnetic bias on the inertial mass of the crash sensor below its nominal level, thereby decreasing its acceleration threshold--and, hence, increasing its sensitivity--as desired. The crash sensor is thereafter able to close in response to further acceleration inputs which are nonetheless below its nominal acceleration threshold, whereupon an increased current flows through the squib and the air bag is deployed.
In contrast to other known control circuits, the Inose et al. circuit "fails soft" in the event of a failure of the low-threshold arming sensor, i.e., a strong vehicle deceleration exceeding the crash sensor's nominal acceleration threshold will still cause the crash sensor to close, thereby and deploying the air bag. Unfortunately, however, the Inose et al. circuit cannot deploy the air bag in the event that the high-threshold crash sensor fails in the "open" position.
U.S. Pat. No. 3,890,594, issued June 17, 1975 to Hosaka et al. also teaches a control circuit for a vehicle safety device. A pair of crash sensors are normally urged in the open position by the delivery of a current through coils integral therewith, respectively, as controlled by a normally-closed "crush" sensor, e.g., a glass element with a conductive coating. In the event of a vehicle collision, the crush sensor opens to cut the current to the coils, thereby reducing the biasing force on the inertial mass of each crash sensor to increase the sensitivity thereof. The crash sensors are thereafter allowed to close in response to the deceleration attendant to the collision. As an alternate embodiment, Hosaka et al. teaches that a normally-open crush sensor may be employed so as to direct a current through the coils in the event of a collision, whereupon the coils generate a magnetic field which tends to urge the crash sensors to close, whereby the sensitivity of the sensors is likewise increased.
However, in either of the embodiments taught by Hosaka et al., in the event that either of the crash sensors thereof fails in the "open" position, the air bag cannot be deployed, and the vehicle passengers are once again placed at substantial risk.